FIG. 1 depicts a schematic diagram of an exemplary wireless local-area network (LAN) 100 in the prior art comprising access point 101 and stations 102-1 through 102-N, wherein N is a positive integer, interconnected as shown. Each station 102-i, wherein i is a member of the set {1, 2, . . . N}, is a device such as a notebook computer, personal digital assistant (PDA), tablet PC, etc. that transmits radio signals to and receives radio signals from other stations in local-area network 100 via access point 101.
Access point 101 and stations 102-1 through 102-N transmit data in units referred to as frames over a shared-communications channel such that if two or more stations (or an access point and a station) transmit frames simultaneously, then one or more of the frames can become corrupted (resulting in a collision). As a result, local-area networks typically employ one or more protocols to ensure that a station or access point can gain exclusive access to the shared-communications channel for an interval of time in order to transmit its frames. Frames transmitted from a station 102-i to access point 101 are referred to as uplink frames, and frames transmitted from access point 101 to a station 102-i are referred to as downlink frames.
In accordance with some protocols (e.g., Institute of Electrical and Electronics Engineers [IEEE] 802.11, etc.), access point 101 periodically broadcasts a special frame called a beacon to all of the stations 102-1 through 102-N. The beacon contains a variety of information that enables stations to establish and maintain communications in an orderly fashion, such as a timestamp, which enables stations to synchronize their local clocks, and signaling information (e.g., channel number, frequency hopping pattern, dwell time, etc.).
A station 102-i can prolong its battery life by powering off its radio when not transmitting or receiving. When a station powers off its radio, the station is said to enter the doze state. A station wakes up from the doze state by powering on its radio to enter the alert state. While a station is in the doze state, it cannot transmit or receive signals, and is said to be asleep. A station that saves battery life by alternating between alert to doze states is said to be in power-save mode, and a station that employs power-save mode is said to be a power-saving station.
While a station 102-i is asleep, access point 101 buffers any downlink frames for station 102-i for eventual delivery when station 102-i wakes up. Three issues therefore arise when a station 102-i is in power-save mode:                (1) When should station 102-i wake up?        (2) How will access point 101 know that station 102-i has awakened?        (3) How will access point 101 know that station 102-i has gone to back to doze state?        
One strategy, which is used in the IEEE 802.11-1999 standard, is for the access point 101 to include periodically in the beacon a Traffic Indication Map (TIM) that identifies which stations in power-save mode have downlink frames waiting for them in access point 101's buffer. When a station wakes up and the TIM indicates that there are frames buffered at access point 101 for the station, the station sends a Power Save (PS) poll frame to access point 101 to request delivery of a buffered frame, and, after receiving and acknowledging the downlink frame, goes back to the doze state. A separate PS poll frame must be transmitted for each downlink frame buffered at access point 101.
In another strategy, known as Automatic Power-Save Delivery (APSD), the delivery of downlink buffered frames can occur automatically—that is, without special signaling frames to notify access point 101 that a station is awake and ready to receive frames.
Another feature of APSD relates to the termination of the awake period, the time interval a power-saving station must remain awake. A power-saving station may stay awake to receive several buffered frames, and goes to back to sleep when it is notified by access point 101.
There are different variations of APSD possible, which differ with respect to when delivery takes place and signaling for the end of a awake period. With the variation that has come to be known as beacon-based APSD, access point 101 periodically includes a Traffic Indication Map in the beacon to identify which stations in power-save mode have downlink frames waiting for them in the access point 101's buffer, as in the 802.11-1999 power-save method. After transmitting a beacon with a TIM, access point 101 transmits its buffered downlink frames.
In accordance with beacon-based APSD, stations in the doze state wake up to receive beacons and check the TIM. If the TIM indicates that there are no buffered downlink frames for a station 102-i, then station 102-i immediately goes back into the doze state; otherwise, station 102-i stays awake to receive the buffered downlink frames from access point 101, and then goes back into power-save mode. In addition, a station in the doze state buffers uplink frames generated by the application layer, and transmits one or more of the buffered uplink frames upon wake-up. Prior to entering power-save mode, a station sends a message to access point 101 that specifies a beacon period for subsequent wake-up (e.g., wake-up every 10 beacons, etc.) and an offset (i.e., phase), thereby identifying the beacons at which the station will wake up. The awake period is terminated by access point 101's notifying the station (e.g., via specially designated bits in the control field(s) of a frame, etc.) that there are no more frames buffered at the access point awaiting transmission.